Focusing and sensing apparatus, methods, and systems

ABSTRACT

Apparatus, methods, and systems provide focusing, focus-adjusting, and sensing. In some approaches the focus-adjusting includes providing an extended depth of focus greater than a nominal depth of focus. In some approaches the focus-adjusting includes focus-adjusting with a transformation medium, where the transformation medium may include an artificially-structured material such as a metamaterial.

TECHNICAL FIELD

The application discloses apparatus, methods, and systems that may relate to electromagnetic responses that include focusing, focus-adjusting, and sensing.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a first configuration of a focusing structure and a focus-adjusting structure.

FIG. 2 depicts a first coordinate transformation.

FIG. 3 depicts a second configuration of a focusing structure and a focus-adjusting structure.

FIG. 4 depicts a second coordinate transformation.

FIG. 5 depicts a focusing structure and a focus-adjusting structure with a spatial separation.

FIG. 6 depicts a focusing structure and a focus-adjusting structure without a spatial separation.

FIG. 7 depicts a first process flow.

FIG. 8 depicts a second process flow.

FIG. 9 depicts a system that includes a focus-adjusting unit and a controller.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

Transformation optics is an emerging field of electromagnetic engineering. Transformation optics devices include lenses that refract electromagnetic waves, where the refraction imitates the bending of light in a curved coordinate space (a “transformation” of a flat coordinate space), e.g. as described in A. J. Ward and J. B. Pendry, “Refraction and geometry in Maxwell's equations,” J. Mod. Optics 43, 773 (1996), J. B. Pendry and S. A. Ramakrishna, “Focusing light using negative refraction,” J. Phys. [Cond. Matt.] 15, 6345 (2003), D. Schurig et al, “Calculation of material properties and ray tracing in transformation media,” Optics Express 14, 9794 (2006) (“D. Schurig et al (1)”), and in U. Leonhardt and T. G. Philbin, “General relativity in electrical engineering,” New J. Phys. 8, 247 (2006), each of which is herein incorporated by reference. The use of the term “optics” does not imply any limitation with regards to wavelength; a transformation optics device may be operable in wavelength bands that range from radio wavelengths to visible wavelengths.

A first exemplary transformation optics device is the electromagnetic cloak that was described, simulated, and implemented, respectively, in J. B. Pendry et al, “Controlling electromagnetic waves,” Science 312, 1780 (2006); S. A. Cummer et al, “Full-wave simulations of electromagnetic cloaking structures,” Phys. Rev. E 74, 036621 (2006); and D. Schurig et al, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314, 977 (2006) (“D. Schurig et al (2)”); each of which is herein incorporated by reference. See also J. Pendry et al, “Electromagnetic cloaking method,” U.S. patent application Ser. No. 11/459,728, herein incorporated by reference. For the electromagnetic cloak, the curved coordinate space is a transformation of a flat space that has been punctured and stretched to create a hole (the cloaked region), and this transformation corresponds to a set of constitutive parameters (electric permittivity and magnetic permeability) for a transformation medium wherein electromagnetic waves are refracted around the hole in imitation of the curved coordinate space.

A second exemplary transformation optics device is illustrated by embodiments of the electromagnetic compression structure described in J. B. Pendry, D. Schurig, and D. R. Smith, “Electromagnetic compression apparatus, methods, and systems,” U.S. patent application Ser. No. 11/982,353; and in J. B. Pendry, D. Schurig, and D. R. Smith, “Electromagnetic compression apparatus, methods, and systems,” U.S. patent application Ser. No. 12/069,170; each of which is herein incorporated by reference. In embodiments described therein, an electromagnetic compression structure includes a transformation medium with constitutive parameters corresponding to a coordinate transformation that compresses a region of space intermediate first and second spatial locations, the effective spatial compression being applied along an axis joining the first and second spatial locations. The electromagnetic compression structure thereby provides an effective electromagnetic distance between the first and second spatial locations greater than a physical distance between the first and second spatial locations.

A third exemplary transform optics device is illustrated by embodiments of the electromagnetic cloaking and/or translation structure described in J. T. Kare, “Electromagnetic cloaking apparatus, methods, and systems,” U.S. patent application Ser. No. 12/074,247; and in J. T. Kare, “Electromagnetic cloaking apparatus, methods, and systems,” U.S. patent application Ser. No. 12/074,248; each of which is herein incorporated by reference. In embodiments described therein, an electromagnetic translation structure includes a transformation medium that provides an apparent location of an electromagnetic transducer different then an actual location of the electromagnetic transducer, where the transformation medium has constitutive parameters corresponding to a coordinate transformation that maps the actual location to the apparent location. Alternatively or additionally, embodiments include an electromagnetic cloaking structure operable to divert electromagnetic radiation around an obstruction in a field of regard of the transducer (and the obstruction can be another transducer).

Additional exemplary transformation optics devices are described in D. Schurig et al, “Transformation-designed optical elements,” Opt. Exp. 15, 14772 (2007); M. Rahm et al, “Optical design of reflectionless complex media by finite embedded coordinate transformations,” Phys. Rev. Lett. 100, 063903 (2008); and A. Kildishev and V. Shalaev, “Engineering space for light via transformation optics,” Opt. Lett. 33, 43 (2008); each of which is herein incorporated by reference.

In general, for a selected coordinate transformation, a transformation medium can be identified wherein electromagnetic waves refract as if propagating in a curved coordinate space corresponding to the selected coordinate transformation. Constitutive parameters of the transformation medium can be obtained from the equations: {tilde over (∈)}^(i′j′)=|det(Λ)|⁻¹Λ_(i) ^(i′)Λ_(j) ^(j′)∈^(ij)   (1) {tilde over (μ)}^(i′j′)=|det(Λ)|⁻¹Λ_(i) ^(i′)Λ_(j) ^(j′)μ^(ij)   (2) where {tilde over (∈)} and {tilde over (μ)} are the permittivity and permeability tensors of the transformation medium, ∈ and μ are the permittivity and permeability tensors of the original medium in the untransformed coordinate space, and

$\begin{matrix} {\Lambda_{i}^{i^{\prime}} = \frac{\partial x^{i^{\prime}}}{\partial x^{i}}} & (3) \end{matrix}$ is the Jacobian matrix corresponding to the coordinate transformation. In some applications, the coordinate transformation is a one-to-one mapping of locations in the untransformed coordinate space to locations in the transformed coordinate space, and in other applications the coordinate transformation is a many-to-one mapping of locations in the untransformed coordinate space to locations in the transformed coordinate space. Some coordinate transformations, such as many-to-one mappings, may correspond to a transformation medium having a negative index of refraction. In some applications, only selected matrix elements of the permittivity and permeability tensors need satisfy equations (1) and (2), e.g. where the transformation optics response is for a selected polarization only. In other applications, a first set of permittivity and permeability matrix elements satisfy equations (1) and (2) with a first Jacobian Λ, corresponding to a first transformation optics response for a first polarization of electromagnetic waves, and a second set of permittivity and permeability matrix elements, orthogonal (or otherwise complementary) to the first set of matrix elements, satisfy equations (1) and (2) with a second Jacobian Λ′, corresponding to a second transformation optics response for a second polarization of electromagnetic waves. In yet other applications, reduced parameters are used that may not satisfy equations (1) and (2), but preserve products of selected elements in (1) and selected elements in (2), thus preserving dispersion relations inside the transformation medium (see, for example, D. Schurig et al (2), supra, and W. Cai et al, “Optical cloaking with metamaterials,” Nature Photonics 1, 224 (2007), herein incorporated by reference). Reduced parameters can be used, for example, to substitute a magnetic response for an electric response, or vice versa. While reduced parameters preserve dispersion relations inside the transformation medium (so that the ray or wave trajectories inside the transformation medium are unchanged from those of equations (1) and (2)), they may not preserve impedance characteristics of the transformation medium, so that rays or waves incident upon a boundary or interface of the transformation medium may sustain reflections (whereas in general a transformation medium according to equations (1) and (2) is substantially nonreflective). The reflective or scattering characteristics of a transformation medium with reduced parameters can be substantially reduced or eliminated by a suitable choice of coordinate transformation, e.g. by selecting a coordinate transformation for which the corresponding Jacobian Λ (or a subset of elements thereof) is continuous or substantially continuous at a boundary or interface of the transformation medium (see, for example, W. Cai et al, “Nonmagnetic cloak with minimized scattering,” Appl. Phys. Lett. 91, 111105 (2007), herein incorporated by reference).

In general, constitutive parameters (such as permittivity and permeability) of a medium responsive to an electromagnetic wave can vary with respect to a frequency of the electromagnetic wave (or equivalently, with respect to a wavelength of the electromagnetic wave in vacuum or in a reference medium). Thus, a medium can have constitutive parameters ∈₁, μ₁, etc. at a first frequency, and constitutive parameters ∈₂, μ₂, etc. at a second frequency; and so on for a plurality of constitutive parameters at a plurality of frequencies. In the context of a transformation medium, constitutive parameters at a first frequency can provide a first response to electromagnetic waves at the first frequency, corresponding to a first selected coordinate transformation, and constitutive parameters at a second frequency can provide a second response to electromagnetic waves at the second frequency, corresponding to a second selected coordinate transformation; and so on: a plurality of constitutive parameters at a plurality of frequencies can provide a plurality of responses to electromagnetic waves corresponding to a plurality of coordinate transformations. In some embodiments the first response at the first frequency is substantially nonzero (i.e. one or both of ∈₁ and μ₁ is substantially non-unity), corresponding to a nontrivial coordinate transformation, and a second response at a second frequency is substantially zero (i.e. ∈₂ and μ₂ are substantially unity), corresponding to a trivial coordinate transformation (i.e. a coordinate transformation that leaves the coordinates unchanged); thus, electromagnetic waves at the first frequency are refracted (substantially according to the nontrivial coordinate transformation), and electromagnetic waves at the second frequency are substantially nonrefracted. Constitutive parameters of a medium can also change with time (e.g. in response to an external input or control signal), so that the response to an electromagnetic wave can vary with respect to frequency and/or time. Some embodiments may exploit this variation with frequency and/or time to provide respective frequency and/or time multiplexing/demultiplexing of electromagnetic waves. Thus, for example, a transformation medium can have a first response at a frequency at time t₁, corresponding to a first selected coordinate transformation, and a second response at the same frequency at time t₂, corresponding to a second selected coordinate transformation. As another example, a transformation medium can have a response at a first frequency at time t₁, corresponding to a selected coordinate transformation, and substantially the same response at a second frequency at time t₂. In yet another example, a transformation medium can have, at time t₁, a first response at a first frequency and a second response at a second frequency, whereas at time t₂, the responses are switched, i.e. the second response (or a substantial equivalent thereof) is at the second frequency. The second response can be a zero or substantially zero response. Other embodiments that utilize frequency and/or time dependence of the transformation medium will be apparent to one of skill in the art.

Constitutive parameters such as those of equations (1) and (2) (or reduced parameters derived therefrom) can be realized using artificially-structured materials. Generally speaking, the electromagnetic properties of artificially-structured materials derive from their structural configurations, rather than or in addition to their material composition.

In some embodiments, the artificially-structured materials are photonic crystals. Some exemplary photonic crystals are described in J. D. Joannopoulos et al, Photonic Crystals: Molding the Flow of Light, 2^(nd) Edition, Princeton Univ. Press, 2008, herein incorporated by reference. In a photonic crystals, photonic dispersion relations and/or photonic band gaps are engineered by imposing a spatially-varying pattern on an electromagnetic material (e.g. a conducting, magnetic, or dielectric material) or a combination of electromagnetic materials. The photonic dispersion relations translate to effective constitutive parameters (e.g. permittivity, permeability, index of refraction) for the photonic crystal. The spatially-varying pattern is typically periodic, quasi-periodic, or colloidal periodic, with a length scale comparable to an operating wavelength of the photonic crystal.

In other embodiments, the artificially-structured materials are metamaterials. Some exemplary metamaterials are described in R. A. Hyde et al, “Variable metamaterial apparatus,” U.S. patent application Ser. No. 11/355,493; D. Smith et al, “Metamaterials,” International Application No. PCT/US2005/026052; D. Smith et al, “Metamaterials and negative refractive index,” Science 305, 788 (2004); D. Smith et al, “Indefinite materials,” U.S. patent application Ser. No. 10/525,191; C. Caloz and T. Itoh, Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications, Wiley-Interscience, 2006; N. Engheta and R. W. Ziolkowski, eds., Metamaterials: Physics and Engineering Explorations, Wiley-Interscience, 2006; and A. K. Sarychev and V. M. Shalaev, Electrodynamics of Metamaterials, World Scientific, 2007; each of which is herein incorporated by reference.

Metamaterials generally feature subwavelength elements, i.e. structural elements with portions having electromagnetic length scales smaller than an operating wavelength of the metamaterial, and the subwavelength elements have a collective response to electromagnetic radiation that corresponds to an effective continuous medium response, characterized by an effective permittivity, an effective permeability, an effective magnetoelectric coefficient, or any combination thereof. For example, the electromagnetic radiation may induce charges and/or currents in the subwavelength elements, whereby the subwavelength elements acquire nonzero electric and/or magnetic dipole moments. Where the electric component of the electromagnetic radiation induces electric dipole moments, the metamaterial has an effective permittivity; where the magnetic component of the electromagnetic radiation induces magnetic dipole moments, the metamaterial has an effective permeability; and where the electric (magnetic) component induces magnetic (electric) dipole moments (as in a chiral metamaterial), the metamaterial has an effective magnetoelectric coefficient. Some metamaterials provide an artificial magnetic response; for example, split-ring resonators (SRRs)—or other LC or plasmonic resonators—built from nonmagnetic conductors can exhibit an effective magnetic permeability (c.f. J. B. Pendry et al, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Micro. Theo. Tech. 47, 2075 (1999), herein incorporated by reference). Some metamaterials have “hybrid” electromagnetic properties that emerge partially from structural characteristics of the metamaterial, and partially from intrinsic properties of the constituent materials. For example, G. Dewar, “A thin wire array and magnetic host structure with n<0,” J. Appl. Phys. 97, 10Q101 (2005), herein incorporated by reference, describes a metamaterial consisting of a wire array (exhibiting a negative permeability as a consequence of its structure) embedded in a nonconducting ferrimagnetic host medium (exhibiting an intrinsic negative permeability). Metamaterials can be designed and fabricated to exhibit selected permittivities, permeabilities, and/or magnetoelectric coefficients that depend upon material properties of the constituent materials as well as shapes, chiralities, configurations, positions, orientations, and couplings between the subwavelength elements. The selected permittivites, permeabilities, and/or magnetoelectric coefficients can be positive or negative, complex (having loss or gain), anisotropic, variable in space (as in a gradient index lens), variable in time (e.g. in response to an external or feedback signal), variable in frequency (e.g. in the vicinity of a resonant frequency of the metamaterial), or any combination thereof. The selected electromagnetic properties can be provided at wavelengths that range from radio wavelengths to infrared/visible wavelengths; the latter wavelengths are attainable, e.g., with nanostructured materials such as nanorod pairs or nano-fishnet structures (c.f. S. Linden et al, “Photonic metamaterials: Magnetism at optical frequencies,” IEEE J. Select. Top. Quant. Elect. 12, 1097 (2006) and V. Shalaev, “Optical negative-index metamaterials,” Nature Photonics 1, 41 (2007), both herein incorporated by reference). An example of a three-dimensional metamaterial at optical frequencies, an elongated-split-ring “woodpile” structure, is described in M. S. Rill et al, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nature Materials advance online publication, May 11, 2008, (doi:10.1038/nmat2197).

While many exemplary metamaterials are described as including discrete elements, some implementations of metamaterials may include non-discrete elements or structures. For example, a metamaterial may include elements comprised of sub-elements, where the sub-elements are discrete structures (such as split-ring resonators, etc.), or the metamaterial may include elements that are inclusions, exclusions, layers, or other variations along some continuous structure (e.g. etchings on a substrate). Some examples of layered metamaterials include: a structure consisting of alternating doped/intrinsic semiconductor layers (cf. A. J. Hoffman, “Negative refraction in semiconductor metamaterials,” Nature Materials 6, 946 (2007), herein incorporated by reference), and a structure consisting of alternating metal/dielectric layers (cf. A. Salandrino and N. Engheta, “Far-field subdiffraction optical microscopy using metamaterial crystals: Theory and simulations,” Phys. Rev. B 74, 075103 (2006); and Z. Jacob et al, “Optical hyperlens: Far-field imaging beyond the diffraction limit,” Opt. Exp. 14, 8247 (2006); each of which is herein incorporated by reference). The metamaterial may include extended structures having distributed electromagnetic responses (such as distributed inductive responses, distributed capacitive responses, and distributed inductive-capacitive responses). Examples include structures consisting of loaded and/or interconnected transmission lines (such as microstrips and striplines), artificial ground plane structures (such as artificial perfect magnetic conductor (PMC) surfaces and electromagnetic band gap (EGB) surfaces), and interconnected/extended nanostructures (nano-fishnets, elongated SRR woodpiles, etc.).

With reference now to FIG. 1, an illustrative embodiment is depicted that includes a focusing structure 110 and a focus-adjusting structure 120. This and other drawings, unless context dictates otherwise, can represent a planar view of a three-dimensional embodiment, or a two-dimensional embodiment (e.g. in FIG. 1 where the transducers are positioned inside a metallic or dielectric slab waveguide oriented normal to the page). The focusing structure receives input electromagnetic energy, depicted as solid rays 102; in this example, the input electromagnetic energy radiates from an electromagnetic source 101 positioned on an optical axis 112 of the focusing structure (the use of a ray description, in FIG. 1 and elsewhere, is a heuristic convenience for purposes of visual illustration, and is not intended to connote any limitations or assumptions of geometrical optics; further, the elements depicted in FIG. 1 can have spatial dimensions that are variously less than, greater than, or comparable to a wavelength of interest). The solid rays 103 represent output electromagnetic energy from the focusing structure. This output electromagnetic energy is received by the focus-adjusting structure 120, which influences the propagation of the output electromagnetic energy (e.g. by refracting the rays 103, as depicted). The dashed rays 104 represent the “nominal” propagation of the output electromagnetic energy, i.e. the propagation that would occur in the absence of the focus-adjusting structure. As the dashed rays indicate, the focusing structure 110 provides a nominal focusing region 130 having a nominal depth of focus 132; in this example, the nominal focusing region 130 is depicted as a slab having a thickness equal to the nominal depth of focus 132 and centered on a nominal focal plane 134 (where the dashed rays converge). The focus-adjusting structure 120 influences the propagation of the output electromagnetic energy to provide an actual focusing region 140 different than the nominal focusing region 130, the actual focusing region having an actual depth of focus 142 which is an extended depth of focus greater than the nominal depth of focus; in this example, the actual focusing region 140 is depicted as a slab having a thickness equal to the actual depth of focus 142 and centered on an actual focal plane 144. Embodiments optionally include one or more electromagnetic sensors (schematically depicted as ellipses 150) positioned within the actual focusing region. The dashed ellipse 152 represents a nominal sensor positioning, i.e. a sensor positioning within the nominal focusing region 130 that could apply in the absence of the focus-adjusting structure (in the present example, sensors are positioned only along the optical axis 112 of the focusing structure, but this is not intended to be limiting). As the figure demonstrates, the actual depth of focus may, in some embodiments, accommodate more sensors than the nominal depth of focus.

The focusing structure 110 is depicted in FIG. 1 as having a lens-like shape, but this is a schematic illustration and is not intended to be limiting. In various embodiments, focusing structures can include reflective structures (e.g. parabolic dish reflectors), refractive structures (e.g. dielectric, microwave, gradient index, or metamaterial lenses), diffractive structures (e.g. Fresnel zone plates), antenna structures (e.g. antenna director elements, antenna arrays), waveguiding structures (e.g. waveguides, transmission lines, and coherent bundles thereof) and various combinations, assemblies, portions, and hybrids thereof (such as an optical assembly or a refractive-diffractive lens). In general, a focusing structure provides a nominal focusing region wherein electromagnetic energy coupled to the focusing structure is nominally substantially concentrated (i.e. wherein electromagnetic energy coupled to the focusing structure is substantially concentrated in the absence of a focus-adjusting structure; the influence of the focus-adjusting structure is discussed below). In some embodiments, the nominal focusing region may be a planar or substantially planar slab (e.g. 130 in FIG. 1) having a thickness corresponding to a nominal depth of focus of the focusing structure (e.g. 132 in FIG. 1) and centered on a nominal focal plane (e.g. 134 in FIG. 1). In other embodiments, the nominal focusing region may be a non-planar region, e.g. a cylindrically-, spherically-, ellipsoidally-, or otherwise-curved slab having a thickness corresponding to a nominal depth of focus of the focusing structure, and the non-planar region may enclose a non-planar focal surface (such as a Petzval, sagittal, or transverse focal surface). In some embodiments the focusing structure defines an optical axis (such as that depicted as element 112 in FIG. 1) as a symmetry or central axis of the focusing structure, and the optical axis provides an axial direction (such as the axial unit vector 160 in FIG. 1), with transverse directions defined perpendicular thereto (such as the transverse unit vectors 161 and 162 in FIG. 1). More generally, one may define an axial direction corresponding to the nominal depth of focus (with transverse directions perpendicular thereto), so that the nominal depth of focus is equivalent to a nominal axial dimension of the nominal focusing region. This is consistent with FIG. 1, where the nominal focusing region is a planar slab, and the axial direction corresponds to a unit vector normal to the slab. Where the nominal focusing region is curved, the axial direction can vary along the transverse extent of the focusing region. For example, where the nominal focusing region is a cylindrically- or spherically-curved slab, the axial direction corresponds to a radius unit vector (and the transverse directions correspond to height/azimuth unit vectors or azimuth/zenith unit vectors, respectively); where the nominal focusing region is an otherwise-curved slab, the axial direction corresponds to a vector locally normal to the slab surface (and the transverse directions correspond to orthogonal unit vectors locally tangent to the slab surface).

In some embodiments the nominal depth of focus of a focusing structure may be related to an f-number f/# of the focusing structure, and/or to a resolution length l in a transverse direction of the nominal focusing region. The f-number can correspond to a ratio of focal length to aperture diameter for the focusing structure, and may (inversely) characterize the convergence of electromagnetic energy towards the nominal focusing region; moreover, the convergence can correspond to a ratio of nominal depth of focus to transverse resolution length; so the following general relation may apply for the focusing structure:

$\begin{matrix} {f/{\left. \# \right.\sim\frac{d_{N}}{l}}} & (4) \end{matrix}$ where d_(N) is the nominal depth of focus. Thus, for a fixed f-number, a larger (smaller) nominal depth of focus corresponds to a larger (smaller) resolution length. This is demonstrated, for example, in FIG. 1, which indicates a resolution length 136 corresponding to a transverse extent of the nominal rays 104 at the surface of the nominal focusing region 130. In some embodiments the resolution length may corresponds to a circle of confusion (CoC) diameter limit for image blur perception, and/or the resolution length may correspond to a transverse extent of (the sensitive area of) an individual sensor or multiplet of sensors, as further discussed below.

In the context of a focus-adjusting structure (e.g. 120 in FIG. 1), one may further define (in addition to a nominal focusing region discussed above) an actual focusing region as a region wherein electromagnetic energy coupled to the focusing structure is substantially concentrated in the presence of the focus-adjusting structure. In some embodiments, the actual focusing region may be a planar or substantially planar slab (e.g. 140 in FIG. 1) having a thickness corresponding to an actual depth of focus of the focusing structure (e.g. 142 in FIG. 1) and centered on an actual focal plane (e.g. 144 in FIG. 1). In other embodiments, the actual focusing region may be a non-planar region, e.g. a cylindrically-, spherically-, ellipsoidally-, or otherwise-curved slab having a thickness corresponding to an actual depth of focus of the focusing structure, and the non-planar region may enclose a non-planar focal surface (such as a Petzval, sagittal, or transverse focal surface). In embodiments where the nominal focusing region and the actual focusing region are substantially parallel (for substantially planar slabs), substantially concentric/confocal (for substantially cylindrically-, spherically, or ellipsoidally-curved slabs), or otherwise substantially co-curved, the axial and transverse directions (defined previously for the nominal focusing region) also apply to the geometry of the actual focusing region; i.e. the axial direction corresponds to both the nominal and axial depths of focus (with transverse directions perpendicular thereto), and the actual depth of focus is equivalent to a actual axial dimension of the actual focusing region as measured along the axial dimension defined previously.

In some embodiments a focus-adjusting structure, such as that depicted in FIG. 1, includes a transformation medium. For example, the ray trajectories 103 in FIG. 1 correspond to a coordinate transformation that is a uniform spatial dilation along the axial direction 160 (within the axial extent of the focus-adjusting structure 120); this coordinate transformation can be used to identify constitutive parameters for a corresponding transformation medium (e.g. as provided in equations (1) and (2), or reduced parameters obtained therefrom) that responds to electromagnetic radiation as in FIG. 1. Explicitly, for the example of FIG. 1, defining z as an untransformed (nominal) focal distance and z′ as a transformed (actual) focal distance (where the distances are measured from the rear vertex 170 along the axial direction 160), the coordinate transformation z′=f (z) is depicted in FIG. 2. The nominal focusing region 130 and the z-position of the nominal focal plane 134 are indicated on the z-axis; the actual focusing region 140, the z′-position of the actual focal plane 144, and the axial extent of the focus-adjusting structure are indicated on the z′-axis. Defining a scale factor

$\begin{matrix} {{s = {\frac{\mathbb{d}z^{\prime}}{\mathbb{d}z} = {f^{\prime}(z)}}},} & (5) \end{matrix}$ the example of FIGS. 1-2 presents a constant scale factor s>1 within the focus-adjusting structure 120, corresponding to a uniform spatial dilation. Supposing that the focus-adjusting structure is surrounded by an ambient isotropic medium with constitutive parameters ∈^(ij)=∈δ^(ij), μ^(ij)=μδ^(ij), the constitutive parameters of the transformation medium are obtained from equations (1) and (2) and are given by (in a basis with unit vectors 161, 162, and 160, respectively, in FIG. 1)

$\begin{matrix} {{\overset{\sim}{ɛ} = {\begin{pmatrix} s^{- 1} & 0 & 0 \\ 0 & s^{- 1} & 0 \\ 0 & 0 & s \end{pmatrix}ɛ}},{\overset{\sim}{\mu} = {\begin{pmatrix} s^{- 1} & 0 & 0 \\ 0 & s^{- 1} & 0 \\ 0 & 0 & s \end{pmatrix}{\mu.}}}} & (6) \end{matrix}$ Thus, the uniform spatial dilation of FIGS. 1-2 corresponds to a transformation medium that is a uniform uniaxial medium.

In some embodiments, the focus-adjusting structure includes a transformation medium that provides a non-uniform spatial dilation. An example is depicted in FIG. 3 and the corresponding coordinate transformation z′=f (z) is depicted in FIG. 4. In FIG. 3, as in FIG. 1, a focusing structure 110 provides a nominal focusing region 130 having a nominal depth of field 132, and the focus-adjusting structure 120 provides an actual focusing region 140 having an actual depth of field 142, where the actual depth of field is an extended depth of field greater than the nominal depth of field. In contrast to FIG. 1, however, the embodiment of FIG. 3 provides an actual focal plane 144 coincident with the nominal focal plane 134; moreover, after exiting the focus-adjusting structure, the actual rays 103 and the nominal rays 104 propagate identically (implying that an optical path length through the focus-adjusting structure is equal to a nominal optical path length where the focus-adjusting structure is replaced by an ambient medium). These attributes are demonstrated in FIG. 4, where the dashed line z′=z intersects z′=f (z) at the position of the focal plane, and at the endpoints of the focus-adjusting structure. The example of FIGS. 3-4 presents a non-uniform scale factor s (the slope of the mapping function z′=f (z)); indeed, the scale factor in this example is variously less than unity (corresponding to a local spatial compression) and greater than unity (corresponding to a local spatial dilation). The constitutive relations are again given by equations (6), where s is variable in the axial direction, and the transformation medium is a non-uniform uniaxial medium.

More generally, embodiments of a focus-adjusting structure, operable to provide an extended depth of focus for output electromagnetic energy greater than the nominal depth of focus, may comprise a transformation medium, the transformation medium corresponding to a coordinate transformation that maps a nominal focusing region (with a nominal depth of focus) to an actual focusing region (with an actual depth of focus greater than the nominal depth of focus); and the constitutive relations of this transformation medium may be implemented with an artificially-structured material (such as a metamaterial), as described previously. In some embodiments, the coordinate transformation from the nominal focusing region to the actual focusing region includes a spatial dilation along an axial direction of the nominal focusing region, and a scale factor of the spatial dilation (within the actual focusing region) may correspond to a ratio of the actual depth of focus to the nominal depth of focus. This is consistent with FIGS. 2 and 4, where the slope triangle 200, indicating the scale factor on the focal plane, is similar or substantially similar to a triangle with a base 132 and a height 142. Just as the axial direction can vary along a transverse extent of the nominal focusing region, the direction of the spatial dilation can vary as well. Thus, for example, a substantially cylindrically- or spherically-curved actual focusing region may be a (uniform or non-uniform) radial dilation of a substantially cylindrically- or spherically-curved nominal focusing region; a substantially ellipsoidally-curved actual focusing region may be a (uniform or non-uniform) confocal dilation of a substantially ellipsoidally-curved nominal focusing region; etc.

In some embodiments, where the focusing structure defines an f-number f/# as discussed previously, the influence of the focus-adjusting structure provides a modified relationship (as compared to equation (4)) between the f-number, the nominal depth of focus, and the resolution length l. Namely, some embodiments provide the relation

$\begin{matrix} {{{f/{\left. \# \right.\sim\frac{1}{s}}}\frac{d_{A}}{l}},} & (7) \end{matrix}$ where d_(A) is the actual depth of focus and s is a scale factor for a spatial dilation applied along the axial direction. The f-number is here defined independently of the focus-adjusting structure: it is a ratio of the nominal focal path length to aperture diameter for the focusing structure (however, some embodiments provide an actual focal path length equal or substantially equal to the nominal focal path length, notwithstanding that the actual focal distance may be substantially different than the nominal focal distance). Combining equations (4) and (7) recovers the relation d_(A)˜sd_(N) discussed in the preceding paragraph.

The focus-adjusting structure 120 is depicted in FIGS. 1 and 3 as a planar slab, but this is a schematic illustration and is not intended to be limiting. In various embodiments the focus-adjusting structure can be a cylindrically-, spherically-, or ellipsoidally-curved slab, or any other slab- or non-slab-like structure configured to receive the output electromagnetic energy and provide an extended depth of focus greater than the nominal depth of focus. In some embodiments, such as that depicted in FIG. 5, the focus-adjusting structure 120 and the focusing structure 110 may have a spatial separation, defining an intermediate region 500 between the structures; in other embodiments, such as that depicted in FIG. 6, the focus-adjusting structure 120 and the focusing structure 110 may define a composite or contiguous unit. Embodiments may define an input surface region (e.g. region 510 in FIGS. 5 and 6) as a surface region of the focus-adjusting structure that receives output electromagnetic radiation from the focusing structure, and this input surface region may be substantially nonreflective of the output electromagnetic radiation. For example, where the focus-adjusting structure is a transformation medium, equations (1) and (2) generally provide a medium that is substantially nonreflective. More generally, the input surface region may be substantially nonreflective by virtue of a substantial impedance-matching to the adjacent medium. When the focusing structure and a focus-adjusting structure that are spatially separated, the adjacent medium corresponds to the intermediate region (e.g. 500 in FIG. 5). When the focusing structure and a focus-adjusting structure are adjacent, the adjacent medium corresponds to an output surface region 600 (e.g. as depicted in FIG. 6) of the focusing structure.

With impedance-matching, a wave impedance of the input surface region is substantially equal to a wave impedance of the adjacent medium. The wave impedance of an isotropic medium is

$\begin{matrix} {Z_{0} = \sqrt{\frac{\mu}{ɛ}}} & (8) \end{matrix}$ while the wave impedance of a generally anisotropic medium is a tensor quantity, e.g. as defined in L. M. Barkovskii and G. N. Borzdov, “The impedance tensor for electromagnetic waves in anisotropic media,” J. Appl. Spect. 20, 836 (1974) (herein incorporated by reference). In some embodiments an impedance-matching is a substantial matching of every matrix element of the wave impedance tensor (i.e. to provide a substantially nonreflective interface for all incident polarizations); in other embodiments an impedance-matching is a substantial matching of only selected matrix elements of the wave impedance tensor (e.g. to provide a substantially nonreflective interface for a selected polarization only). In some embodiments, the adjacent medium defines a permittivity ∈₁ and a permeability μ₁, where either or both parameters may be substantially unity or substantially non-unity; the input surface region defines a permittivity ∈₂ and a permeability μ₂, where either or both parameters may be substantially unity or substantially non-unity; and the impedance-matching condition implies

$\begin{matrix} {\frac{ɛ_{2}}{ɛ_{1}} \cong \frac{\mu_{2}}{\mu_{1}}} & (9) \end{matrix}$ where ∈₂ and μ₂ may be tensor quantities. Defining a surface normal direction and a surface parallel direction (e.g. depicted as elements 521 and 522, respectively, in FIGS. 5 and 6), some embodiments provide a input surface region that defines: a surface normal permittivity ∈₂ ^(⊥) corresponding to the surface normal direction and a surface parallel permittivity ∈₂ ^(∥) corresponding to the surface parallel direction; and/or a surface normal permeability μ₂ ^(⊥) corresponding to the surface normal direction and a surface parallel permeability μ₂ ^(∥) corresponding to the surface parallel direction; and the impedance-matching condition may imply (in addition to equation (9)) one or both of the following conditions:

$\begin{matrix} {{\frac{ɛ_{2}^{\bot}}{ɛ_{1}} \cong \frac{ɛ_{1}}{ɛ_{2}^{\parallel}}},{\frac{\mu_{2}^{\bot}}{\mu_{1}} \cong {\frac{\mu_{1}}{\mu_{2}^{\parallel}}.}}} & (10) \end{matrix}$ Where the input surface region is a curved surface region (e.g. as in FIG. 6), the surface normal direction and the surface parallel direction can vary with position along the input surface region.

Some embodiments provide one or more electromagnetic sensors positioned within the actual focusing region of the focusing structure. In general, electromagnetic sensors, such as those depicted FIG. 1 and in other embodiments, are electromagnetic devices having a detectable response to received or absorbed electromagnetic energy. Electromagnetic sensors can include antennas (such as wire/loop antennas, horn antennas, reflector antennas, patch antennas, phased array antennas, etc.), solid-state photodetectors (such as photodiodes, CCDs, and photoresistors), vacuum photodetectors (such as phototubes and photomultipliers) chemical photodetectors (such as photographic emulsions), cryogenic photodetectors (such as bolometers), photoluminescent detectors (such as phosphor powders or fluorescent dyes/markers), MEMS detectors (such as microcantilever arrays with electromagnetically responsive materials or elements) or any other devices operable to detect and/or transduce electromagnetic energy. Some embodiments include a plurality of electromagnetic sensors positioned within the actual focusing region. A first example is a multiplet of sensors operable at a corresponding multiplet of wavelengths or wavelength bands, i.e. a first sensor operable at a first wavelength/wavelength band, a second sensor operable at a second wavelength/wavelength band, etc. A second example is a focal plane array of sensors or sensor multiplets (e.g. a Bayer or Foveon sensor). A third example is a phased array of antennas. The plurality of sensors can be axially distributed (as in FIG. 1); for example, the extended depth of focus may admit a plurality of parallel focal plane sensor arrays. As discussed earlier, a transverse extent of the sensitive area of a sensor (or sensor multiplet) can provide a resolution length in the transverse direction (e.g. 136 in FIG. 1), and may bear a relation to the depth of focus (as in equations (4) and (7)).

In some embodiments the focus-adjusting structure provides an extended depth of focus for output electromagnetic energy at a selected frequency/frequency band and/or a selected polarization. The selected frequency or frequency band may be selected from a range that includes radio frequencies, microwave frequencies, millimeter- or submillimeter-wave frequencies, THz-wave frequencies, optical frequencies (e.g. variously corresponding to soft x-rays, extreme ultraviolet, ultraviolet, visible, near-infrared, infrared, or far infrared light), etc. The selected polarization may be a particular TE polarization (e.g. where the electric field is in a particular direction transverse to the axial direction, as with s-polarized electromagnetic energy), a particular TM polarization (e.g. where the magnetic field is in a particular direction transverse to the axial direction, as with p-polarized electromagnetic energy), a circular polarization, etc. (other embodiments provide an extended depth of focus for output electromagnetic energy that is substantially the same extended depth of focus for any polarization, e.g. for unpolarized electromagnetic energy).

In other embodiments the focus-adjusting structure provides a first extended depth of focus for output electromagnetic energy at a first frequency and a second extended depth of focus for output electromagnetic energy at a second frequency, where the second extended depth of focus may be different than or substantially equal to the first extended depth of focus. For embodiments that recite first and second frequencies, the first and second frequencies may be selected from the frequency categories in the preceding paragraph. Moreover, for these embodiments, the recitation of first and second frequencies may generally be replaced by a recitation of first and second frequency bands, again selected from the above frequency categories. These embodiments providing a focus-adjusting structure operable at first and second frequencies may include a transformation medium having an adjustable response to electromagnetic radiation. For example, the transformation medium may have a response to electromagnetic radiation that is adjustable (e.g. in response to an external input or control signal) between a first response and a second response, the first response providing the first extended depth of focus for output electromagnetic energy at the first frequency, and the second response providing the second extended depth of focus for output electromagnetic energy at the second frequency. A transformation medium with an adjustable electromagnetic response may be implemented with variable metamaterials, e.g. as described in R. A. Hyde et al, supra. Other embodiments of a focus-adjusting structure operable at first and second frequencies may include transformation medium having a frequency-dependent response to electromagnetic radiation, corresponding to frequency-dependent constitutive parameters. For example, the frequency-dependent response at a first frequency may provide a first extended depth of focus for output electromagnetic energy at the first frequency, and the frequency-dependent response at a second frequency may provide second extended depth of focus for output electromagnetic energy at the second frequency. A transformation medium having a frequency-dependent response to electromagnetic radiation can be implemented with artificially-structured materials such as metamaterials; for example, a first set of metamaterial elements having a response at the first frequency may be interleaved with a second set of metamaterial elements having a response at the second frequency.

In some embodiments the focusing structure provides a first nominal depth of focus for output electromagnetic energy at a first frequency and a second nominal depth of focus for output electromagnetic energy at a second frequency, where the second nominal depth of focus may be different than or substantially equal to the first nominal depth of focus. Examples of focusing structures providing different first and second nominal depths of focus include: dichroic lenses or mirrors, frequency-selective surfaces, diffractive gratings; metamaterials having a frequency-dependent response; etc.; or, generally, any focusing structure having a chromatic dispersion or aberration. A focusing structure that provides first and second nominal depths of focus for output electromagnetic energy at first and second frequencies can be combined with a focus-adjusting structure that provides first and second extended depths of focus for output electromagnetic energy at the first and second frequencies. A particular embodiment provides different first and second nominal depths of focus but substantially equal first and second extended depths of focus (thus, for example, compensating for a chromatic aberration of the focusing structure).

An illustrative embodiment is depicted as a process flow diagram in FIG. 7. Flow 700 optionally includes operation 710—deflecting an electromagnetic wave, whereby the electromagnetic wave converges towards a nominal focusing region having a nominal axial dimension. For example, a focusing structure, such as that depicted as element 110 in FIGS. 1 and 3, deflects input electromagnetic energy 102, whereby output electromagnetic energy 103 converges towards a nominal focusing region 130. Operation 710 optionally includes sub-operation 712—deflecting a first component of the electromagnetic wave at a first frequency, whereby the first component converges toward a first nominal focusing subregion within the nominal focusing region, the first nominal focusing subregion having a first nominal axial subdimension—and sub-operation 714—deflecting a second component of the electromagnetic wave at a second frequency, whereby the second electromagnetic wave converges toward a second nominal focusing subregion within the nominal focusing region, the second nominal focusing subregion having a second nominal axial subdimension. For example, a focusing structure may provide a first nominal depth of focus for output electromagnetic energy at a first frequency and a second nominal depth of focus for output electromagnetic energy at a second frequency (e.g. where the focusing structure has a chromatic dispersion or aberration ). Flow 700 includes operation 720—substantially-nonreflectively receiving the electromagnetic wave that converges towards the nominal focusing region. For example, a focus-adjusting structure, such as that depicted as element 120 in FIGS. 5 and 6, may include an input surface region 510 that is substantially nonreflective of input electromagnetic energy incident upon the input surface region from an adjacent region (e.g. where the input surface region is substantially impedance-matched to the adjacent region). Flow 700 further includes operation 730—spatially dilating the electromagnetic wave along a direction corresponding to the nominal axial dimension, thereby providing an actual focusing region having an actual axial dimension greater than the nominal axial dimension. For example, a focus-adjusting structure, such as that depicted as element 120 in FIGS. 1 and 3, may receive output electromagnetic energy 103 and influence the propagation thereof, whereby the output electromagnetic energy converges towards an actual focusing region 140 instead of a nominal focusing region 130; and the focus-adjusting structure may include a transformation medium corresponding to a coordinate transformation that includes a spatial dilation, e.g. as depicted in FIGS. 2 and 4. Operation 730 optionally includes sub-operation 732—spatially dilating a first component of the electromagnetic wave at a first frequency, thereby providing a first actual focusing subregion within the actual focusing region, the first actual focusing subregion having a first actual axial subdimension greater than the nominal axial dimension—and sub-operation 734—spatially dilating a second electromagnetic wave at a second frequency, thereby providing a second actual focusing subregion within the actual focusing region, the second actual focusing subregion having a second actual axial subdimension greater than the nominal axial dimension. For example, a focus-adjusting structure may provide a first extended depth of focus for output electromagnetic energy at a first frequency and a second extended depth of focus for output electromagnetic energy at a second frequency, where the second extended depth of focus may be different than or substantially equal to the first extended depth of focus; and the focus-adjusting structure operable at first and second frequencies may include a transformation medium having an adjustable response to electromagnetic radiation, or a transformation medium having a frequency-dependent response to electromagnetic radiation. Flow 700 optionally further includes operation 740—sensing the electromagnetic wave at one or more locations within the actual focusing region. For example, one or more electromagnetic sensors, such as those depicted as elements 150 in FIG. 1, may be positioned within an actual focusing region 140 to detect/receive/absorb the output electromagnetic energy 103.

Another illustrative embodiment is depicted as a process flow diagram in FIG. 8. Flow 800 includes operation 810—identifying a nominal focusing region for a focusing structure, the nominal focusing region having a nominal axial dimension. For example, a nominal focusing region is depicted as region 130 in FIGS. 1 and 3. The nominal focusing region may correspond to a region centered on a nominal focal plane (or non-planar nominal focal surface) having a thickness corresponding to a nominal depth of focus. Operation 810 optionally includes sub-operation 812—for electromagnetic waves at a first frequency, identifying a first nominal focusing subregion within the nominal focusing region for the focusing structure, the first nominal focusing subregion having a first nominal axial subdimension-and sub-operation 814—for electromagnetic waves at a second frequency, identifying a second nominal focusing subregion within the nominal focusing region for the focusing structure, the second nominal focusing subregion having a second nominal axial subdimension. For example, the focusing structure may have a chromatic dispersion or aberration, whereby the nominal focusing region depends upon the frequency of the input electromagnetic energy. Flow 800 further includes operation 820—determining electromagnetic parameters for a spatial region containing the nominal focusing region, the electromagnetic parameters providing an actual focusing region having an actual axial dimension greater than the nominal axial dimension; where the electromagnetic parameters include (1) axial electromagnetic parameters and (2) transverse electromagnetic parameters that inversely correspond to the axial electromagnetic parameters. For example, the spatial region can be a volume that encloses a focus-adjusting structure, such as that depicted as element 120 in FIGS. 1 and 3, and the determined electromagnetic parameters are the electromagnetic parameters of the focus-adjusting structure. The focus-adjusting structure may include a transformation medium, where the determined electromagnetic parameters satisfy or substantially satisfy equations (1) and (2), as described above; or, the determined electromagnetic parameters may be reduced parameters (as discussed earlier) where the corresponding non-reduced parameters satisfy equations (1) and (2). In some embodiments, the determining of the electromagnetic parameters includes: determining a coordinate transformation (such as those depicted in FIGS. 2 and 4); then determining electromagnetic parameters for a corresponding transformation medium (e.g. with equations (1) and (2)); then, optionally, reducing the electromagnetic parameters (e.g. to at least partially substitute a magnetic response for an electromagnetic response, or vice versa, as discussed above). Operation 820 optionally includes sub-operation 822—for electromagnetic waves at a first frequency, determining a first subset of the electromagnetic parameters providing a first actual focusing subregion within the actual focusing region, the first actual focusing subregion having a first actual axial subdimension greater than the nominal axial dimension—and sub-operation 824—for electromagnetic waves at a second frequency, determining a second subset of the electromagnetic parameters providing a second actual focusing subregion within the actual focusing region, the second actual focusing subregion having a second actual axial subdimension greater than the nominal axial dimension. For example, the determined electromagnetic parameters may be the electromagnetic parameters of a focus-adjusting structure having a first extended depth of focus for output electromagnetic energy at a first frequency and a second extended depth of focus for output electromagnetic energy at a second frequency. The focus-adjusting structure may include a transformation medium having an adjustable response to electromagnetic radiation, e.g. adjustable between a first response, corresponding to the first subset of the electromagnetic parameters, and a second response, corresponding to the second subset of the electromagnetic parameters. Or, the focus-adjusting structure may include a transformation medium having a frequency-dependent response to electromagnetic radiation, corresponding to frequency-dependent constitutive parameters, so that the first and second subsets of the electromagnetic parameters are values of the frequency-dependent constitutive parameters at the first and second frequencies, respectively. Flow 800 optionally further includes operation 830—selecting one or more positions of one or more electromagnetic sensors within the spatial region. For example, electromagnetic sensors may be positioned in a phased array, a focal plane array, an axially-distributed arrangement, etc. Flow 800 optionally further includes operation 840—configuring an artificially-structured material having an effective electromagnetic response that corresponds to the electromagnetic parameters in the spatial region. For example, the configuring may include configuring the structure(s) and/or the materials that compose a photonic crystal or a metamaterial. Operation 840 optionally includes determining an arrangement of a plurality of electromagnetically responsive elements having a plurality of individual responses, the plurality of individual responses composing the effective electromagnetic response. For example, the determining may include determining the positions, orientations, and individual response parameters (spatial dimensions, resonant frequencies, linewidths, etc.) of a plurality of metamaterial elements such as split-ring resonators, wire or nanowire pairs, etc. Operation 840 optionally includes configuring at least one electromagnetically-responsive structure to arrange a plurality of distributed electromagnetic responses, the plurality of distributed electromagnetic responses composing the effective electromagnetic response. For example, the configuring may include configuring the distribution of loads and interconnections on a transmission line network, configuring an arrangement of layers in a layered metamaterial, configuring a pattern of etching or deposition (as with a nano-fishnet structure), etc.

With reference now to FIG. 9, an illustrative embodiment is depicted as a system block diagram. The system 900 includes a focusing unit 910 optionally coupled to a controller unit 940. The focusing unit 910 may include a focusing structure such as that depicted as element 110 in FIGS. 1 and 3. The focusing structure may be a variable or adaptive focusing structure, such as an electro-optic lens, a liquid or liquid-crystal lens, a mechanically-adjustable lens assembly, a variable metamaterial lens, or any other variable or adaptive focusing structure responsive to one or more control inputs to vary or adapt one or more focusing characteristics (focal length, aperture size, nominal depth of focus, operating frequency/frequency band, operating polarization, etc.) of the focusing structure; and the controller unit 940 may include control circuitry that provides one or more control inputs to the variable or adaptive focusing structure. The system 900 further includes a focus-adjusting unit 920 coupled to the controller unit 940. The focusing-adjusting unit 920 may include a focus-adjusting structure such as that depicted as element 120 in FIGS. 1 and 3. The focus-adjusting structure may be a variable focus-adjusting structure, such as a variable metamaterial responsive to one or more control inputs to vary one or more focus-adjusting characteristics (extended depth of focus, operating frequency/frequency band, operating polarization, effective coordinate transformation for a transformation medium, etc.); and the controller unit 940 may include control circuitry that provides one or more control inputs to the variable focus-adjusting structure. The controller unit 940 may include circuitry for coordinating or synchronizing the operation of the focusing unit 910 and the focus-adjusting unit 920; for example, the controller unit 940 may vary one or more focusing characteristics of a focusing structure (e.g. vary an operating frequency of the focusing structure from a first frequency to a second frequency) and correspondingly vary one or more focus-adjusting characteristics of a focus-adjusting structure (e.g. vary an operating frequency of the focus-adjusting structure from a first frequency to a second frequency). The system 900 optionally further includes a sensing unit 930 that may include one or more sensors, such as those depicted as elements 150 in FIG. 1, and associated circuitry such as receiver circuitry, detector circuitry, and/or signal processing circuitry. The sensing unit 930 is optionally coupled to the controller unit 940, and in some embodiments the controller unit 940 includes circuitry responsive to sensor data (from the sensor unit 930) to vary the focusing characteristics of a focusing structure and/or vary the focus-adjusting characteristics of a focus-adjusting structure. As a first example, the controller unit 940 may include circuitry responsive to the sensor data to identify a frequency of received energy, adjust the focusing unit 910 to an operating frequency substantially equal to the frequency of received energy, and/or adjust the focus-adjusting unit 920 to an operating frequency substantially equal to the frequency of received energy. As a second example, the controller unit may include circuitry responsive to the sensor data to identify an actual focusing region (e.g. depicted as region 140 in FIGS. 1 and 3) and vary the actual focusing region by varying the focusing characteristics of a focusing structure and/or varying the focus-adjusting characteristics of a focus-adjusting structure.

The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams, flowcharts, and/or examples. Insofar as such block diagrams, flowcharts, and/or examples contain one or more functions and/or operations, it will be understood by those within the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof In one embodiment, several portions of the subject matter described herein may be implemented via Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), digital signal processors (DSPs), or other integrated formats. However, those skilled in the art will recognize that some aspects of the embodiments disclosed herein, in whole or in part, can be equivalently implemented in integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more processors (e.g., as one or more programs running on one or more microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of skill in the art in light of this disclosure. In addition, those skilled in the art will appreciate that the mechanisms of the subject matter described herein are capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment of the subject matter described herein applies regardless of the particular type of signal bearing medium used to actually carry out the distribution. Examples of a signal bearing medium include, but are not limited to, the following: a recordable type medium such as a floppy disk, a hard disk drive, a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape, a computer memory, etc.; and a transmission type medium such as a digital and/or an analog communication medium (e.g., a fiber optic cable, a waveguide, a wired communications link, a wireless communication link, etc.).

In a general sense, those skilled in the art will recognize that the various aspects described herein which can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or any combination thereof can be viewed as being composed of various types of “electrical circuitry.” Consequently, as used herein “electrical circuitry” includes, but is not limited to, electrical circuitry having at least one discrete electrical circuit, electrical circuitry having at least one integrated circuit, electrical circuitry having at least one application specific integrated circuit, electrical circuitry forming a general purpose computing device configured by a computer program (e.g., a general purpose computer configured by a computer program which at least partially carries out processes and/or devices described herein, or a microprocessor configured by a computer program which at least partially carries out processes and/or devices described herein), electrical circuitry forming a memory device (e.g., forms of random access memory), and/or electrical circuitry forming a communications device (e.g., a modem, communications switch, or optical-electrical equipment). Those having skill in the art will recognize that the subject matter described herein may be implemented in an analog or digital fashion or some combination thereof.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in any Application Data Sheet, are incorporated herein by reference, to the extent not inconsistent herewith.

One skilled in the art will recognize that the herein described components (e.g., steps), devices, and objects and the discussion accompanying them are used as examples for the sake of conceptual clarity and that various configuration modifications are within the skill of those in the art. Consequently, as used herein, the specific exemplars set forth and the accompanying discussion are intended to be representative of their more general classes. In general, use of any specific exemplar herein is also intended to be representative of its class, and the non-inclusion of such specific components (e.g., steps), devices, and objects herein should not be taken as indicating that limitation is desired.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations are not expressly set forth herein for sake of clarity.

While particular aspects of the present subject matter described herein have been shown and described, it will be apparent to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from the subject matter described herein and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of the subject matter described herein. Furthermore, it is to be understood that the invention is defined by the appended claims. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

With respect to the appended claims, those skilled in the art will appreciate that recited operations therein may generally be performed in any order. Examples of such alternate orderings may include overlapping, interleaved, interrupted, reordered, incremental, preparatory, supplemental, simultaneous, reverse, or other variant orderings, unless context dictates otherwise. With respect to context, even terms like “responsive to,” “related to,” or other past-tense adjectives are generally not intended to exclude such variants, unless context dictates otherwise.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

What is claimed is:
 1. An electromagnetic apparatus, comprising: a focusing structure oriented to receive input electromagnetic energy and having a nominal depth of focus for output electromagnetic energy; and a focus-adjusting structure having an axial direction and a traverse direction disposed to receive the output electromagnetic energy and having electromagnetic parameters selected from a group consisting of permittivity and permeability, wherein the electromagnetic parameters provide an extended depth of focus for the output electromagnetic energy greater than the nominal depth of focus and include: axial electromagnetic parameters; and transverse electromagnetic parameters having an inverse relationship to the axial electromagnetic parameters.
 2. The electromagnetic apparatus of claim 1, wherein the focusing structure includes a refractive structure.
 3. The electromagnetic apparatus of claim 1, wherein the focusing structure includes a reflective structure.
 4. The electromagnetic apparatus of claim 1, wherein the focusing structure includes a diffractive structure.
 5. The electromagnetic apparatus of claim 1, wherein the focusing structure includes at least a portion of an antenna.
 6. The electromagnetic apparatus of claim 1, wherein the extended depth of focus corresponds to an actual focusing region.
 7. The electromagnetic apparatus of claim 6, further comprising: at least one electromagnetic sensor positioned within the actual focusing region.
 8. The electromagnetic apparatus of claim 6, wherein the actual focusing region is at least partially within the focus-adjusting structure.
 9. The electromagnetic apparatus of claim 8, wherein the electromagnetic parameters define an index of refraction having an absolute value less than one within at least a portion of the actual focusing region.
 10. The electromagnetic apparatus of claim 1, wherein the focusing structure defines the axial direction, and the axial electromagnetic parameters correspond to the axial direction.
 11. The electromagnetic apparatus of claim 10, wherein the axial direction corresponds to an optical axis of the focusing structure.
 12. The electromagnetic apparatus of claim 10, wherein the transverse electromagnetic parameters correspond to the transverse direction, the transverse direction being substantially perpendicular to the axial direction.
 13. The electromagnetic apparatus of claim 1, wherein the axial electromagnetic parameters include an axial permittivity.
 14. The electromagnetic apparatus of claim 13, wherein the transverse electromagnetic parameters include a transverse permittivity that is substantially a multiplicative inverse of the axial permittivity.
 15. The electromagnetic apparatus of claim 13, wherein the axial electromagnetic parameters include an axial permeability.
 16. The electromagnetic apparatus of claim 15, wherein the transverse electromagnetic parameters include a transverse permeability that is substantially a multiplicative inverse of the axial permeability.
 17. The electromagnetic apparatus of claim 15, wherein the axial permittivity is substantially equal to the axial permeability.
 18. The electromagnetic apparatus of claim 17, wherein the transverse electromagnetic parameters include a transverse permeability that is substantially a multiplicative inverse of the axial permeability.
 19. The electromagnetic apparatus of claim 1, wherein the focusing structure and the focus-adjusting structure have a spatial separation.
 20. The electromagnetic apparatus of claim 19, wherein the spatial separation defines an intermediate region between the focusing structure and the focus-adjusting structure, and wherein the focus-adjusting structure includes an input surface region that is substantially nonreflective of electromagnetic energy incident upon the input surface region from the intermediate region.
 21. The electromagnetic apparatus of claim 20, wherein a wave impedance of the intermediate region is substantially equal to a wave impedance of the input surface region.
 22. The electromagnetic apparatus of claim 21, wherein the intermediate region defines a first permittivity and a first permeability, the input surface region defines a second permittivity and a second permeability, and a ratio of the second permittivity to the first permittivity is substantially equal to a ratio of the second permeability to the first permeability.
 23. The electromagnetic apparatus of claim 22, wherein the input surface region defines a surface normal direction and a surface parallel direction, the second permittivity includes a surface normal permittivity corresponding to the surface normal direction and a surface parallel permittivity corresponding to the surface parallel direction, and a ratio of the surface normal permittivity to the first permittivity is substantially a multiplicative inverse of a ratio of the surface parallel permittivity to the first permittivity.
 24. The electromagnetic apparatus of claim 23, wherein the second permeability includes a surface normal permeability corresponding to the surface normal direction and a surface parallel permeability corresponding to the surface parallel direction, and a ratio of the surface normal permeability to the first permeability is substantially a multiplicative inverse of a ratio of the surface parallel permeability to the first permeability.
 25. The electromagnetic apparatus of claim 1, wherein the focusing structure and the focus-adjusting structure define a composite unit.
 26. The electromagnetic apparatus of claim 25, wherein the focusing structure includes an output surface region and the focus-adjusting structure includes an input surface region substantially adjoining the output surface region, the input surface region being substantially nonreflective of electromagnetic energy incident upon the input surface region from the output surface region.
 27. The electromagnetic apparatus of claim 26, wherein a wave impedance of the output surface region is substantially equal to a wave impedance of the input surface region.
 28. The electromagnetic apparatus of claim 27, wherein the output surface region defines a first permittivity and a first permeability, the input surface region defines a second permittivity and a second permeability, and a ratio of the second permittivity to the first permittivity is substantially equal to a ratio of the second permeability to the first permeability.
 29. The electromagnetic apparatus of claim 28, wherein the input surface region defines a surface normal direction and a surface parallel direction, the second permittivity includes a surface normal permittivity corresponding to the surface normal direction and a surface parallel permittivity corresponding to the surface parallel direction, and a ratio of the surface normal permittivity to the first permittivity is substantially a multiplicative inverse of a ratio of the surface parallel permittivity to the first permittivity.
 30. The electromagnetic apparatus of claim 29, wherein the second permeability includes a surface normal permeability corresponding to the surface normal direction and a surface parallel permeability corresponding to the surface parallel direction, and a ratio of the surface normal permeability to the first permeability is substantially a multiplicative inverse of a ratio of the surface parallel permeability to the first permeability.
 31. The electromagnetic apparatus of claim 1, wherein the focus-adjusting structure includes an artificially-structured material having an effective electromagnetic response that corresponds to the electromagnetic parameters.
 32. The electromagnetic apparatus of claim 31, wherein the artificially-structured material includes a photonic crystal.
 33. The electromagnetic apparatus of claim 31, wherein the artificially-structured material includes a metamaterial.
 34. The electromagnetic apparatus of claim 33, wherein the metamaterial includes at least one electromagnetically-responsive structure having a plurality of distributed electromagnetic responses, the plurality of distributed electromagnetic responses composing the effective electromagnetic response.
 35. The electromagnetic apparatus of claim 34, wherein the distributed electromagnetic responses include distributed inductive responses.
 36. The electromagnetic apparatus of claim 34, wherein the distributed electromagnetic responses include distributed capacitive responses.
 37. The electromagnetic apparatus of claim 34, wherein the distributed electromagnetic responses include distributed inductive-capacitive responses.
 38. The electromagnetic apparatus of claim 34, wherein the at least one electromagnetically-responsive structure includes transmission lines.
 39. The electromagnetic apparatus of claim 33, wherein the metamaterial includes a plurality of electromagnetically responsive elements disposed at a plurality of spatial locations and having a plurality of individual responses, the plurality of individual responses composing the effective electromagnetic response.
 40. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include discrete circuit elements.
 41. The electromagnetic apparatus of claim 40, wherein the discrete circuit elements include inductors.
 42. The electromagnetic apparatus of claim 40, wherein the discrete circuit elements include capacitors.
 43. The electromagnetic apparatus of claim 40, wherein the discrete circuit elements include semiconductor devices.
 44. The electromagnetic apparatus of claim 43, wherein the semiconductor devices include diodes.
 45. The electromagnetic apparatus of claim 43, wherein the semiconductor devices include transistors.
 46. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include integrated circuit elements.
 47. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include printed circuit elements.
 48. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include metallic structures.
 49. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include LC resonators.
 50. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include plasmonic resonators.
 51. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include nanostructures.
 52. The electromagnetic apparatus of claim 51, wherein the nanostructures include nanorods.
 53. The electromagnetic apparatus of claim 52, wherein the nanorods are paired nanorods.
 54. The electromagnetic apparatus of claim 52, wherein the nanorods are interconnected to compose a fishnet structure.
 55. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include split-ring resonators.
 56. The electromagnetic apparatus of claim 39, wherein the electromagnetically responsive elements include subwavelength elements having spatial extents substantially less than a free space wavelength corresponding to a frequency of the input electromagnetic energy. 